U.S. patent number 4,789,580 [Application Number 06/930,351] was granted by the patent office on 1988-12-06 for process of reducing higher metal oxides to lower metal oxides.
This patent grant is currently assigned to Metallgesellschaft Aktiengesellschaft. Invention is credited to Martin Hirsch, Hermann Lommert, Harry Serbent.
United States Patent |
4,789,580 |
Hirsch , et al. |
December 6, 1988 |
Process of reducing higher metal oxides to lower metal oxides
Abstract
Disclosed is a process to effect a reduction to a desired,
constant degree as exactly as possible and to achieve a low surplus
of carbon. The reduction by treatment with carbonaceous reducing
agents is effected in such a manner that fine-grained solids, which
contain higher metal oxides, are calcined at 800.degree. to
1100.degree. C. with hot gases in which the solids are suspended.
The calcined solids are reduced at a temperature in the range of
from 800.degree. to 1100.degree. C. to form low metal oxides in a
stationary fluidized bed, which is supplied with carbonaceous
reducing agents and oxygen-containing gases. The carbonaceous
reducing agents are supplied to the stationary fluidized bed at
such a rate so as to reduce the higher metal oxides to low metal
oxides, while maintaining the reduction temperature in the
stationary fluidized bed and insuring that the discharged matter
has the desired carbon content. The stationary fluidized bed
exhaust gas is supplied as secondary gas to the calcining step, and
fuel is supplied to the calcining step at a rate such that the
total of the heat generated by the combustion of the fuel and of
the heat suplied by the exhaust gas provides the heat which is
required for calcination.
Inventors: |
Hirsch; Martin (Friedrichsdorf,
DE), Lommert; Hermann (New Isenburg, DE),
Serbent; Harry (Hanau am Main, DE) |
Assignee: |
Metallgesellschaft
Aktiengesellschaft (Frankfurt am Main, DE)
|
Family
ID: |
6286069 |
Appl.
No.: |
06/930,351 |
Filed: |
November 13, 1986 |
Foreign Application Priority Data
|
|
|
|
|
Nov 15, 1985 [DE] |
|
|
3540541 |
|
Current U.S.
Class: |
75/500;
423/DIG.16; 423/148; 423/605; 423/632; 423/49; 423/594.19 |
Current CPC
Class: |
C22B
5/10 (20130101); C22B 47/00 (20130101); C22B
23/023 (20130101); Y10S 423/16 (20130101) |
Current International
Class: |
C22B
23/02 (20060101); C22B 23/00 (20060101); C22B
5/10 (20060101); C22B 5/00 (20060101); C22B
47/00 (20060101); C01G 045/02 (); C01G
049/04 () |
Field of
Search: |
;423/168,138,148,632,633,634,594,605,DIG.16,592,49
;75/26,35,36,82,91 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0035653 |
|
Jun 1973 |
|
AU |
|
4628485 |
|
Aug 1984 |
|
AU |
|
0955384 |
|
Jan 1950 |
|
FR |
|
1564579 |
|
Aug 1969 |
|
FR |
|
2318938 |
|
Jul 1975 |
|
FR |
|
0008400 |
|
Oct 1912 |
|
GB |
|
Primary Examiner: McFarlane; Anthony
Attorney, Agent or Firm: Felfe & Lynch
Claims
We claim:
1. A process of reducing the higher valencies of a metal oxide to
lower valencies of the metal oxide comprising:
(a) calcining in a circulating fluidized bed reactor fine-grained
solids which contain metal oxides of higher valency at at a
temperature of 800.degree. to 1100.degree. C. under oxidizing
conditions with hot gases, in which the solids are suspended;
(b) reducing the calcined solids at a reduction temperature in a
rnage of from 800.degree. to 1100.degree. C. in a stationary
fluidized bed which is supplied with a carbonaceous reducing agent
and oxygen-containing gases to reduce the higher metal oxides to
lower metal oxides;
(c) supplying the carbonaceous reducing agent to the stationary
fluidized bed and removing discharge matter from said bed, the
carbonaceous reducing agent being supplied at a rate effective to
reduce the higher valency metal oxide to a lower valency metal
oxide, to maintain the reduction temperature, and to maintain a
carbon content in the discharged matter;
(d) removing exhaust gas from the stationary fluidized bed and
introducing the exhaust gas as a secondary gas in the circulating
fluidized bed in the calcining step; and
(e) supplying fuel to the calcining step (a) and substantailly
completely combusting the fuel and the exhaust gas, the fuel being
supplied at such a rate that the total of the heat generated by the
substantially complete combustion of said fuel and of the heat
supplied to the calcining step by the combustion of the exhaust gas
from step (d) is sufficient to effect the calcination.
2. The process of claim 1, wherein a suspension is discharged from
the circulating fluidized bed reactor and is supplied to a
separator wherein solids are separated, at least a portion of the
separated solids is recycled to said reactor, and the exhaust gas
is supplied to a suspension heat exchanger for drying and
preheating the solids which contain the higher metal oxide.
3. The process of claim 1, wherein the exhaust gas of the
stationary fluidized bed is passed through a separator ot separate
solids therefrom before being supplied to the calcining step, and
the solids separated from the exhaust gas are recycled to the
stationary fluidized bed.
4. The process of claim 1, wherein the solids are iron-nickel ores
containing oxides of nickel and iron and the carbonaceous reducing
agent is supplied in step (c) to the stationary fluidized bed at
such a rate so as to effect a reduction of the higher valency iron
oxide substantially to an FeO state, a reduction of the nickel
oxide, and the mantenance of the reduction temperature in step (b)
and of the content of carbon not in excess of 2% by weight in the
matter discharged.
5. The process of claim 4, wherein tne discharged matter is
processed further in a molten state to form metallic iron in an
amount sufficient for making an iron-nickel alloy, and wherein any
remaining iron present in said discharge matter is slagged.
6. The process of claim 1, wherein the higher valency metal oxide
is a maganese oxide containing material and carbonaceous reducing
agent is supplied in step (c) to the stationary fluidized bed at
such a rate so as to effect a reduction of maganese oxide of higher
valency substantially to the MnO state, and a maintenance of the
reduction temperature in step (b) and to minimize the amount of
carbon contained in the discharged matter.
7. The process of claim 1 wherein the carbonaceous reducing agent
which us supplied to the reducing step (b) is a solid.
Description
BACKGROUND OF THE INVENTION
The present invention is in a process for reducing higher metal
oxides to lower metal oxides by treatment with a carbonaceous
reducing agent.
Ores which contain metals, such as Fe, Ni, Mn, in the form of
higher oxides must sometimes be subjected to a reducing treatment
to obtain the metals in a lower oxide form. This is particularly
required in processes of producing iron-nickel alloys from
iron-nickel ores.
Poor ores, such as lateritic ores, must increasingly be used to
meet the demand of industry for nickel, particularly in alloys with
iron. Most of the poor ores contain Fe and Ni in a ratio such that
a complete reduction of both metals, and a separation of the gangue
in a molten state as slag, would result in a ferroalloy which is so
poor in nickel that it would not be commercially acceptable.
For instance, in an ore containing 30% Fe and 2% Ni the ratio of Fe
to Ni is 15:1. However, the Fe:Ni ratio in commercial ferroalloys
is not in excess of 4:1, which means that they contain at least 20%
nickel.
For this reason the processing of such ores includes a preliminary
reduction, by which they are reduced, as closely as possible, to an
FeO state and a succeeding melting process, in which metallic iron
is produced by a further reduction only in that amount which is
permissible for the desired ferroalloy. The remaining iron oxide is
slagged.
On a commercial scale, the preliminary reduction is effected in a
rotary kiln and coal is used as the reducing agent. A problem
arising in connection with the preliminary reduction in a rotary
kiln is that the iron oxide must be reduced by the preliminary
reduction exactly to a prescribed degree and that the discharged
material must contain surplus solid carbon only in an amount which
is still permissible for the further reduction in the melting
process to the desired content of metallic iron. Formation of
metallic iron by the preliminary reduction must be avoided even
though the degree of reduction achieved by the preliminary
reduction in the rotary kiln is subject to relatively strong
fluctuations. As such, the preliminary reduction is not effected as
far as to the FeO state but, for the sake of precaution, only to a
much higher degree of oxidation so that a larger reduction work
must be performed in the melting process, which is effected in
electric furnaces in most cases. As a result, the overall process
becomes more expensive. Additionally, control of further reduction
during the melting process is difficult because the degree of
oxidation and the carbon content of the matter discharged from the
rotary kiln often fluctuate, even in the case of small kilns.
Such a process has been described in TMS-AIME Paper Selection,
Paper No. 74-40, pages 1-23.
Another case relates to the reduction of ores which contain higher
manganese oxides to be reduced to lower manganese oxides.
SUMMARY OF THE INVENTION
It is an object of the invention to effect a reduction of higher
metal oxides to the highest possible degree to lower metal oxides
having the desired oxidation number, which degree should be as
constant as possible, and to minimize the carbon content of the
reduced matter which is discharged or to provide only a constant,
small content of surplus carbon therein.
This and other objects are accomplished in accordance with the
invention. In the process of the invention, fine-grained solids
which contain higher metal oxides are calcined by a treatment at
800.degree. to 1100.degree. C. with hot gases, in which the solids
are suspended. The calcined solids are reduced from the higher
metal oxides to lower metal oxides at a temperature in the range of
from 800.degree. to 1100.degree. C. in a stationary fluidized bed.
The stationary fluidized bed is supplied with carbonaceous reducing
agents and oxygen-containing gases. The carbonaceous reducing agent
is supplied to the fluidized bed at such a rate that the carbon
which is supplied is effective to reduce the higher metal oxides to
low metal oxides, to maintain the reduction temperature, and to
maintain the desired carbon content in the matter discharged. The
exhaust gas from the stationary fluidized bed is used as a
secondary gas in the calcining step. Fuel is supplied to the system
for the calcining step at such a rate that the total of the heat
generated by the combustion of such fuel and of the heat supplied
to the calcining step by the exhaust gas from the stationary
fluidized bed will be sufficient to effect the calcination.
The solids have a particle size below 3 mm.
By the calcination, water of crystallization is eliminated,
carbonates are decomposed with formation of CO.sub.2, and any
moisture which is present is evaporated. The calcination is
effected under oxidizing conditions. The hot gases may be produced
by a combustion of solid, liquid and gaseous fuels.
The calcination may be effected in a stationary fluidized bed or a
circulating fluidized bed or by a different process in which the
solids are suspended in a gas stream. The raw materials may be
dried before they are calcined. That drying may be effected with
the waste heat from the calcining step. In that case water will be
evaporated without a consumption of carbon. Besides, the water
vapor need not be heated to the much higher temperature used for
calcination and the waste heat will be utilized in a favorable
manner. The dried solids may be heated further before they are
supplied to the calcining step and such further heating may result
in a preliminary calcination to a certain degree.
The solids withdrawn from the calcining step are subjected to a
preliminary reduction in a stationary (orthodox) fluidized bed. A
stationary fluidized bed is a fluidized bed in which a dense phase
is separated by a distinct density step from the overlying
dust-laden space and the two states of distribution are separated
by a defined boundary layer.
The oxygen-containing gases are supplied as fluidizing gas to the
stationary fluidized bed at such a rate that the carbonaceous
reducing agent is virtually completely gasified or is gasified to
such a degree that the discharged matter has a desired content of
surplus carbon. The oxygen-containing gases generally consist of
air.
The actual amount of supplied carbonaceous reducing agent in the
stationary fluidized bed is calculated based on the heat balance
(maintain the reduction temperature) as well as the amount required
for the reduction step and the desired carbon content in the
discharged matter. The reduction step is performed under the
required reducing conditions so that the waste gas from the
stationary fluidized bed contains CO and H.sub.2 in such amount
which is necessary in accordance with the thermodynamic
conditions.
In a preferred embodiment, the calcining step is effected in a
circulating fluidized bed. The suspension discharged from the
fluidized bed reactor is supplied to a separator, at least one
partial stream of the separated solids is recycled to the reactor,
and the exhaust gas is supplied to suspension heat exchangers for
drying and preheating the solids which contain higher metal oxides.
The system of the circulating fluidized bed consists of a fluidized
bed reactor, a separator and a recycling line for recycling solids
from the separator to the fluidized bed reactor.
Whereas an orthodox fluidized bed constitutes a dense phase, which
is separated by a distinct density step from the overlying gas
space, the fluidized bed in the fluidized bed reactor of the
circulating fluidized bed contains states of distribution having no
defined boundary layer. There is no density step between a dense
phase and an overlying gas space but the solids concentration in
the reactor gradually decreases from bottom to top. If the
operating conditions are defined by the Froude and Archimedes
numbers, the following ranges will be obtained: ##EQU1## wherein
##EQU2##
In said equations
u=the relative gas velocity in m/sec
Ar=the Archimedes number
Fr=the Froude number
.rho..sub.g =the density of the gas in kg/m.sup.3
.rho..sub.k =the density of the solid particle in kg/m.sup.3
d.sub.k =the diameter of the spherical particle in m
.nu.=the kinematic viscosity in m.sup.2 /sec
g=the acceleration due to gravity in m/sec.sup.2
In order to form a circulating fluidized bed, the solids entrained
by the gases from the fluidized bed reactor are recycled to the
fluidized bed reactor so that the quantity of solids circulated per
hour is at least five times the weight of the solids contained in
the shaft of the reactor. At the rate at which solids are charged,
solids are withdrawn from the system of the circulating fluidized
bed and are supplied to the stationary fluidized bed. The
circulating fluidized bed will effect a calcination at a high
throughput rate and a combustion of the fuel to a high degree and
owing to the multistage combustion will ensure that the exhaust gas
has only low contents of CO and NO.sub.x.
In a preferred embodiment, the exhaust gas coming from the
stationary fluidized bed and used as secondary gas in the calcining
step, is passed through a separator before being supplied to the
calcining step. The separated solids are recycled to the stationary
fluidized bed. The dust-collecting separator suitably consists of a
cyclone. In that case a recycling of solids from the reducing stage
to the oxidizing stage will be substantially avoided.
In a preferred embodiment, solid carbonaceous reducing agents are
supplied to the stationary fluidized bed wherein the reduction
occurs. The supply of solid fuel will result in an improved
distribution in the fluidized bed and will permit a very exact
maintenance of a uniform content of surplus carbon in the
discharged matter.
In a preferred embodiment, iron-nickel ores are charged and
carbonaceous reducing agent is supplied in step (c) to the
stationary fluidized bed at such a rate as to effect a reduction of
the higher iron oxides approximately to an FeO state, a reduction
of the nickel oxides, and a maintenance of the reduction
temperature in the bed during reduction and of a content of surplus
carbon not in excess of 2% by weight in the matter discharged. The
discharged matter is processed further in a molten state with
formation of metallic iron in the amount required for the desired
iron-nickel alloy. The remaining iron content is slagged.
In a further embodiment, materials which contain manganese oxides
are processed and carbonaceous reducing agent is supplied to the
stationary fluidized bed at a rate so as to effect a reduction of
the higher manganese oxides, approximately to the MnO state and a
maintenance of the reduction temperature in the bed and to minimize
the surplus carbon contained in the discharged matter
The various features of novelty which characterize the invention
are pointed out with particularity in the claims annexed to and
forming a part of this specification. For a better understanding of
the invention, its operating advantages and specific objects
obtained by its use, reference should be had to the accompanying
drawings and descriptive matter in which there is illustrated and
described a preferred embodiment of the invention .
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE schematically depicts the process of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the FIGURE, ore 1 is charged, by a screw conveyor 2,
into a venturilike suspension dryer 3, in which the ore is
suspended in a gas stream. The suspension is then conveyed through
line 4 to a separator 5. The gas is purified in an electrostatic
precipitator 6 and is then discharged as exhaust gas 7. The
collected solids are fed by a screw conveyor 7a into line 8. A
partial stream from line 8 is supplied through line 9 to a
calcining system. The calcining system includes a circulating
fluidized bed which consists of a fluidized bed reactor 10, a
recycling cyclone 11 and a recycling line 12.
That portion of the solids from line 8 which is not fed through
line 9 into the calcining system is supplied through line 13 to a
preheater 14 and is suspended therein in a gas stream. The
preheated suspension is subsequently supplied through line 15 to a
separator 16. The solids collected in separator 16 are supplied
through line 17 to the reactor 10. The off gas from the separator
16 flows into suspension dryer 3.
Fluidizing air 18 is supplied to the lower portion of the reactor
10. Secondary air 19 and coal 20 are supplied to the reactor 10 on
a higher level. A gas-solids suspension is formed in and fills the
entire fluidized bed reactor 10. The gas-solids suspension passes
out from the top of the reactor 10 and is supplied in line 21 to a
recycling cyclone 11 wherein the suspension is separated into
solids and gas. The gas flows into the preheater 14 and the
separated solids enter the recycling line 12, which contains a
siphon trap 12a. Trap 12a is supplied at its bottom with fluidizing
air at a low rate (not shown).
From the trap 12a, a portion of the calcined solids flows through a
controllable valve 22 and a line 23 to the reactor 24, which
contains the stationary fluidized bed, in which reduction is
effected. Fluidizing air is blown through line 25 into the lower
portion of the reactor 24, which is supplied with coal through line
26. The dust-laden exhaust gas from the reactor 24 is conducted in
line 27 to the separator 28. The therein collected solids are
recycled through line 29 to the stationary fluidized bed in reactor
24. The exhaust gas from the separator 28 is supplied through line
30 to the fluidized bed reactor 10 and enters the same above the
lines 12, 19 and 20 and below the line 17. The reduced material is
discharged through line 31. Calcined solids may be supplied through
line 32 to the reactor 24 which contains the stationary fluidized
bed.
EXAMPLE
The reference numerals are consistent with those used in the
FIGURE.
The screw conveyor 2 delivered 100,000 kg/h of the lateritic ore
into the system. The lateritic Ni ore had the following contents
based on dry ore:
______________________________________ Fe.sub.2 O.sub.3 20%;
SiO.sub.2 33.0% NiO 2%; MgO 26.9% CaCO.sub.3 6.8%; water of
hydration 9.9%; moisture of wet ore 13.7%
______________________________________
The fluidized bed reactor 10 was 3.7 m in diameter and had a height
of 16 m. A temperature of 900.degree. C. was maintained in the
reactor during operation.
Fluidizing air (line 18) was introduced into reactor 10 at a rate
of 20,000 sm.sup.3 /h Secondary air (line 19) was fed into reactor
10 at a rate of 22,600 sm.sup.3 /h. The coal (line 20) was fed into
the reactor at a feed rate of 4,260 kg/h. The coal 20
contained:
81.8% C;
2.6% H;
5.6% 0;
3.3% ash;
6.7% moisture;
and had a lower heating value H.sub.u of 7,043 kcal/kg. The volume
of the exhaust gas from reactor 10 (line 21) was 58,600 sm.sup.3 /h
at a temperature of 900.degree. C.
The feed of line 8 was split so that 60% of solids from line 8 pass
through the valves into line 9 and the remaining 40% of solids from
line 8 were diverted into line 13.
The fluidized bed reactor 24 was 3 m in diameter and had a height
of 2.5 m. A temperature of 900.degree. C. was maintained in the
reactor. In the operation of reactor 24, 6,030 sm.sup.3 /h were
introduced as fluidizing air (line 25) and coal (line 26) was
introduced at a rate of 3,430 kg/h. 9,430 sm.sup.3 /h of gas was
exhausted from reactor 24 (line 27). The exhaust gas had the
following composition:
18.2% CO;
17.6% CO.sub.2 ;
6.3% H.sub.2 ;
7.4% H.sub.2 O; and
50.5% N.sub.2.
The material discharged through line 31 amounted to 72,650 kg/h and
had the following composition:
21.4% FeO; 39.2% SiO.sub.2
1.9% Ni; 32.0% MgO
1.3% C. 4.5% CaO
The system exhaust gas (line 7) was 82,000 sm.sup.3 /h and had the
following makeup:
13.7% CO.sub.2 ;
37.8% H.sub.2 O
46.7% N.sub.2 ; and
1.8% O.sub.2 at a temperature of 140.degree. C.
Thus the process of the invention provides an advantage in that the
calcination is effected with very high economy and with production
of a substantially completely burnt exhaust gas having a low
pollutant content and that a reduced product is obtained which has
been reduced to an exactly controlled, uniform degree and has an
exactly defined, uniform content of surplus carbon or a zero carbon
content
In the application to the reduction of iron-nickel ores, the iron
oxides may substantially be reduced to FeO whereas a formation of
metallic iron is avoided. The carbon content of the discharged
matter may be minimized or may be maintained at the low, absolutely
uniform level that is required for a reduction by which only a
small amount of metallic iron is formed in the melting process. For
this reason the rate at which carbon is supplied to the electric
furnace can be exactly controlled.
It will be understood that the specification and examples are
illustrative but not limitative of the present invention and that
other embodiments within the spirit and scope of the invention will
suggest themselves to those skilled in the art.
* * * * *